Patent application title: MAGNETRON

Abstract:

A magnetron has an anode 3 surrounding a tubular hollow cathode 4 which
contains a heater 9. The cathode is supported by radial arms at each end.
At one end of the cathode, the heater is supplied with one terminal of
its D.C. supply by means of a radial arm 5, which also serves to support
that end of the cathode. The arm has a portion 5a offset towards the
cathode, and a cover of conducting material is interposed between the
heater connection and the adjacent end wall 1 of the vacuum envelope. The
cover may have a folded portion so that it can be carried by the arm.

Claims:

1. A magnetron comprising a cathode, an anode surrounding the cathode, the
region between the anode and the cathode being within a vacuum chamber, a
heater for the cathode having a D.C. supply connection at an end of the
cathode, and a cover of conducting material interposed between the D.C.
supply connection and the adjacent end of the vacuum chamber

2. A magnetron as claimed in claim 1, wherein the cover is supported on an
arm supplying D.C. to the heater.

3. A magnetron as claimed in claim 2, wherein the cover has a folded
region to be securely supported on the arm.

4. A magnetron as claimed in claim 2, wherein the D.C. supply connection
is in a portion of the arm which is offset towards the cathode.

5. A magnetron as claimed in claim 1, wherein the cover is made of nickel
or a nickel alloy.

6. A magnetron as claimed in claim 1, wherein the heater extends along a
hollow interior of the cathode.

7. A magnetron as claimed in claim 6, including a lead which extends from
the heater and which passes through an insulating sleeve in the end of
the cathode.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the priority of Great Britain Application
No. 0805277.1, filed Mar. 20, 2008, the contents of which is incorporated
herein by reference.

BACKGROUND OF THE INVENTION

[0002]This invention relates to magnetrons.

[0003]A known magnetron will now be described with reference to FIGS. 1
and 2 of the drawings. FIG. 1 is a fragmentary sectional view of the
magnetron, taken through the axis of the anode, and FIG. 2 is an enlarged
view of the anode and cathode shown in FIG. 1. Referring to FIG. 1, the
magnetron comprises a vacuum chamber having end walls 1 and 2, which are
at right angles to the axis of the anode 3 and cathode 4 of the
magnetron. There are resonant cavities (not shown) defined in the anode,
or by vanes. A magnetic field is applied normal to the plane of the end
walls 1,2, by an electromagnet or a permanent magnet (not shown).

[0004]The cathode 4 is tubular, and has a heater extending along its axis,
and a D.C. supply to the heater, as well as a high negative voltage for
the cathode, is supplied to the cathode by means of conducting supports
5,6, which extend into an upper region 7 of the magnetron, the interior
of which is within the vacuum envelope. The conducting supports connect
to terminals on a part of the exterior of the upper region that is not
shown. Couplers (not shown) extend into a resonant cavity and withdraw
power into an output section 8, which can be coupled to a waveguide.

[0005]Referring to FIG. 2, which shows the tubular cathode 4 in more
detail, a small DC supply voltage for the heater 9 is provided between
the supports 5,6, and a large pulsed negative DC voltage is applied to
support 6 only. The heater 9 is connected to the end of the tubular
cathode at its right-hand end (as seen in FIG. 2), and the support 6
directly connects to the cathode. At its left-hand end, the heater 9 is
supported in the cathode by an insulating sleeve 10, and is connected to
the support 5.

[0006]The cathode is supported on radial arms 5, 6 that enable the
magnetic field to be applied directly by a separate electromagnet (not
shown). The gap over which the magnetic field is applied is desirably
minimized so the electromagnet is as small as possible and uses least
power. The vacuum gap between the ends of the tubular cathode structure
and the end walls 1,2 of the magnetron has to be sufficient to hold off
the negative voltage, typically 50 KV, that is applied to the cathode
relative to the anode and the magnetron body, including end walls, under
normal working conditions. Experience has shown that the cathode to end
wall gap is not adequate to prevent arcing under all conditions
(particularly when driven with line-type modulators) and very
occasionally this can have serious consequences when the arcing causes
the end wall to puncture. It is believed that in addition to the applied
pulse voltage across the cathode to side wall gap there are RF voltages
picked up from where the cathode supports pass near the anode,
particularly if there is a projection such as if the anode is provided
with a strap 11. These picked-up voltages may be increased by resonances
on the cathode supports or in the space between end plate and anode.

[0007]The heater connection 12 on the cathode is a sharp point, which
further enhances the voltage stress in this area. The result is that the
heater connection can form a seat for arcing, which can confine any
arcing that occurs to the region of the wall 1 that is immediately
adjacent, and thus increase the risk of perforation.

[0008]The Applicants contemplated counteracting this risk by the expedient
of increasing the gap between side wall and cathode but this would mean
the outline of the magnetron would need to change. However, there are
thousands of equipments currently in use, which require the current
profile for the magnetron, so such a modification would be
disadvantageous. It is also undesirable to make anything more than the
minimum change to the interior of the magnetron, as any change risks
upsetting its operation.

SUMMARY OF THE INVENTION

[0009]The invention provides a magnetron comprising a cathode, an anode
surrounding the cathode, the region between the anode and the cathode
being within a vacuum chamber, a heater for the cathode having a D.C.
supply connection at an end of the cathode, and a cover of conducting
material interposed between the D.C. supply connection and the adjacent
end of the vacuum chamber.

[0011]The cover plate could be made of any conductor that is vacuum tube
compatible. Nickel or a nickel alloy would be suitable because of its
availability, ease of machining and ease of joining.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]The invention will now be described in detail, by way of example,
with reference to the accompanying drawings, in which:

[0013]FIG. 1 is a schematic sectional view of a known magnetron, partly
cut away, taken through the axis of the cathode;

[0014]FIG. 2 is a view of a part of the magnetron of FIG. 1 shown in
greater detail;

[0015]FIG. 3 shows a modification according to the invention to one part
of the known magnetron of FIG. 2;

[0016]FIG. 4 is a view taken in the direction of the arrows 4-4 in FIG. 3;

[0017]FIG. 5 is an axial section through a part of a magnetron according
to the invention; and

[0018]FIG. 6 is a perspective view of the cover used in the magnetron
shown in FIG. 5.

[0019]Like parts have been given like reference numerals throughout all
the drawings.

DETAILED DESCRIPTION

[0020]The invention can best be appreciated by considering FIGS. 3 and 4
in conjunction with FIG. 2, which show a modification made in accordance
with the invention to the known magnetron. By comparing FIG. 3 with FIG.
2, it will be seen that, the support arm 5 is provided with a region 5a
which is offset towards the cathode, so that the connection point 12
between the heater and the support arm 5 is displaced nearer to the
cathode than in the prior art magnetron. In addition and in accordance
with the invention, a cover 13 of conducting material is interposed
between the connection point 12 and the adjacent end wall 1 of the vacuum
chamber. The cover is made of nickel, or a nickel alloy, but other
conducting materials could be used if desired. The region of the arm 5
over which the cover extends is in fact formed by two closely spaced
conductors, which diverge in the region of connection point 12 to make
the connection to the end of the heater easier. The upper end of the
cover is folded behind the support 5 in order to provide a secure
anchorage for the cover to be welded or soldered to the arm, although the
cover could if desired be mechanically mounted on the arm.

[0021]There will of course be the same voltage between the cover 13 and
the adjacent wall 1 of the vacuum chamber (since the face of the plate
follows the line of the original support 5), but the sharp point 12 is
now electrostatically shielded, so that there is no tendency for such
arcing as may occur to take place preferentially in the region of the
wall 1 immediately adjacent the connection 12. Such arcing as occurs will
be spread over the surface of the cover and over a corresponding area of
the wall. Thus, the risk of perforation of the wall have been reduced or
eliminated.

[0022]FIGS. 5 and 6 show a practical embodiment of the invention. The
heater 9 terminates in a lead 9a which is surrounded by a collar 14 that
is insulated from the tubular cathode 4 by insulating sleeve 10a and
insulating ring 10b. The support arm 5 is in two parts, 5b and 5c. The
latter is formed by one length of conductor, which is bent into two
closely spaced strands where the parts are joined. The strands are
connected to opposite sides of the end of the heater lead. The cover is
shown in FIG. 6, and it will be seen that the upper part of it is folded
back on itself at 13a, the arm being sandwiched between the front and
folded back section of the cover, and the cover is secured to the arm by
welding or soldering.

[0023]The invention is especially applicable to high power magnetrons,
that is, magnetrons with peak output powers exceeding 1 MW. A typical
range of operating frequencies is from 1 GHz to 20 GHz, the design being
especially suitable for S-band operation, that is, from 2 GHz to 4 GHz.
Such magnetrons are suitable for use in linear accelerators.